1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens. The present invention is suitable for, for example, imaging systems for digital cameras, video cameras, and silver-halide film cameras.
2. Description of the Related Art
A so-called negative-lead zoom lens is known in which a lens unit having a negative refractive power is disposed at a position closest to the object side. The negative-lead zoom lens has a small close-up shooting distance, and the field angle and the back focus can be easily increased. Therefore, the negative-lead zoom lens is often used as a wide field angle imaging lens.
A zoom lens including four lens units, which are a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power arranged in order from the object side, is known as a negative-lead zoom lens for a single-lens reflex camera (see, for example, U.S. Pat. No. 5,710,669 and Japanese Patent Laid-Open Nos. 2006-58584, 2001-42217, 10-325923, and 10-82954).
Each of these documents describes a wide field angle zoom lens including four lens units, that is, a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power arranged in order from the object side to the image side. The wide field angle zoom lens performs zooming by moving each of the lens units.
U.S. Pat. No. 5,710,669 describes a zoom lens having a super wide angle range in which the field angle is about 100° at the wide-angle end.
According to Japanese Patent Laid-Open No. 2006-58584, a first lens unit includes three negative lenses and a positive lens. Thus, a zoom lens having improved optical performance and a super wide angle range in which the imaging field angle is 110° or more is provided.
Recently, there has been an increasing demand for zoom lenses with wide field angle and high image quality for use in digital single-lens reflex cameras.
In general, although the negative-lead zoom lens is advantageous in increasing the field angle, they have a problem that the lens structure is very asymmetric.
Therefore, when each lens unit is moved during zooming, aberrations largely vary due to the asymmetric lens structure, and it is extremely difficult to obtain high optical performance over the entire zooming range.
In particular, in a zoom lens including a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power in order from the object side to the image side, it is difficult to obtain a wide field angle and high optical performance over the entire zooming range unless the first lens unit, which is closest to the object side, has a suitable lens structure.
For example, if an Abbe number or a partial dispersion ratio of a material of each lens included in the first lens unit is not suitable, large chromatic aberration of magnification occurs at the wide-angle end. In addition, the chromatic aberration of magnification largely varies over the entire zooming range, and it is difficult to correct the chromatic aberration of magnification.